Carbon canisters are generally known, and are used for capturing and storing vapor from a fuel tank, which is transferred to the engine through a canister purge valve. Typically, these fuel tanks are open to atmosphere via the carbon canister, thereby keeping the tank pressure near atmospheric pressure at all times. In newer, sealed tank systems, the fuel tank is isolated from the canister, and is capable of storing vapor under pressure, or vacuum, to a set pressure/vacuum level. The amount of fuel in the fuel tank varies as the fuel is consumed during vehicle travel, and during refueling at a service station. During vehicle travel, the amount of fuel in the fuel tank is decreased, creating vacuum pressure in the fuel tank. In a sealed tank system, this vacuum pressure must be relieved to eliminate the possibility of collapsing the tank. Approaches to relieving this vacuum pressure have included a solenoid having a valve which controls the amount of air flow back into the tank under vacuum pressure conditions, and the flow of fuel vapor to the carbon canister.
Some of these canister purge valves use short pulses to open and close the valve rapidly to keep the vapors contained in the tank as much as possible, while also providing vacuum relief. While this approach provides controlled vacuum relief, it does not provide for passive (uncontrolled) vacuum relief, separately from the flow control.
Accordingly, there exists a need for a valve assembly which is able to provide active and passive relief of vacuum pressure in a fuel tank.